Compared to lithium-ion batteries, lithium-air batteries have extremely high energy density. As such, lithium-air batteries may be promising candidates for applications in plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV), where performance and high energy density are desired. However, traditional solvents pose technical barriers to the production of lithium-air batteries as a suitable non-aqueous electrolyte that can conduct both lithium ions to a negative electrode and oxygen to the positive air electrode. For example, carbonate-based solvents are known for use in lithium batteries, however carbonates can be electrochemically oxidized at positive potentials versus the Li+/Li couple, in the absence of oxygen. For example, ethylene carbonate (EC) can be electrochemically oxidized at a potential of 4.8 V vs. Li+/Li in the absence of oxygen. In the presence of oxygen, the chemical and electrochemical oxidation of carbonates is more facile, occurring at potentials lower than 4.8 V vs. Li+/Li. The high vapor pressure of some carbonates at room temperature introduces other issues with regard to the development of air breathing membranes, through which oxygen is supplied from the air to the positive electrodes.
A lithium-air cell typically has a lithium negative electrode and an air positive electrode. Oxygen gas, introduced into the battery through the air cathode, is essentially an unlimited cathode reactant source so that the capacity of the battery is limited by the Li anode. During discharge, when the cell delivers the energy stored to the external load, the external current flows from the positive electrode to the negative electrode. The lithium in the negative electrode loses an electron; and the lithium ion is transported to the positive electrode through the electrolyte sandwiched between the negative electrode and the positive electrode. Meanwhile, at the positive electrode, oxygen is absorbed from atmospheric air and is reduced. As a result of the discharge process, lithium is removed from the negative electrode and lithium oxide is then deposited on the positive electrode. During the charging process, lithium oxide in the positive electrode is decomposed, lithium is deposited back to the negative electrode, and the resulting oxygen is released to the air.
Compared to lithium-ion cells, the lithium-air cell does not require host materials, for insertion or de-insertion of the lithium, at both electrodes. It is the host materials in lithium-ion cells that limit the capacity density and energy density of the cell. For instance, lithium-transition metal oxides are generally used as the host materials in positive electrodes of lithium-ion batteries, and such metal oxides typically have a specific capacity of less than 280 mAh/g. Graphite is generally used in lithium-ion batteries as the negative electrode material. Mesocarbon microbeads (MCMB) are one such graphitic material, and can provide a theoretical capacity of about 372 mAh/g. During the discharge of a lithium-ion cell, lithium is removed from the lithiated negative electrode and is inserted into the lithium-transition metal oxide of the positive electrode. During charge, lithium is removed from the positive electrode material and is re-inserted into the negative electrode material.
As described above, lithium-air batteries are not based on the intercalation mechanism of lithium-ion batteries. The specific capacity of the lithium anode is about 3800 mAh/g, which is about 10 times of the capacity of MCMB used as the negative electrodes for lithium ion batteries. The positive electrode of the lithium-air batteries is basically a conductive porous media without the presence of the host materials for lithium; storing the critical and unlimited component of oxygen in air. Thus, to access the higher capacities of lithium-air cells, solvents that are stable in such cells are required.